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What kinds of effects are present in digital images that are referred to as "noise"?

  • What are the different sources of noise?

  • What causes each type of noise?

  • What are the characteristics of the different types of noise?

  • How do the different types of noise visually manifest in the image? (i.e., do different kinds of noise "look different" is the resulting image, and how would they differ to the eye?)

  • How can you minimize each type?

  • What different post-processing techniques are most appropriate for each different kind?

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2 Answers 2

Noise is often defined as any deviation from a "pure" signal. The signal is taken to be brightness pattern of the image so any variation in the pixel values that represent the image is noise. These variations arise principally due to:

  • Shot noise. The random way photons are emitted from a lightsource causes random variations in image brightness. The fewer photons you have the more this noise is evident. Can be reduced by getting more light onto the sensor.

  • Dark current (thermal) noise. Heat produced by the camera (which being electromagnetic radiation just like light can show up on the sensor). Since it's not part of the scene it's noise. It can be reduced by cooling the sensor, limiting exposure times (the longer the sensor is active for the more it heats up) or shooting a dark frame (i.e. with the shutter closed or lens cap on) to subtract from the original image (some cameras have a setting to automate this).

  • Photo response non uniformity (fixed pattern noise). This arises from imperfections in the silicon that cause pixels to be slightly more or less sensitive than their neighbours. Calibration can reduce PRNU, although it can be dependent on parameters such as exposure time.

  • Read noise. Electrical noise that is generated by the circuitry which reads the values from the sensor pixels. Can be reduced by using a higher ISO (in the case where the signal is not maximised, amplifying the signal prior to readout means read noise is a smaller percentage of the signal) or using a camera with lower read noise. You can look at the shadow noise figures at base ISO to give you an idea of read noise.

  • Quantisation noise. Rounding errors when an analogue signal is converted into a finite set of descrete digital values. Not usually noticeable, can be reduced by using a sensor which stores more bits per pixel e.g. 14 instead of 12.

The following are technically noise but rarely referred to as such:

  • Moire/aliasing. A sort of spatial quantisation noise, aliasing arises due to interference patterns and the fixed spacing of sensor elements. It can be reduced by an anti-aliasing filter (usually fitted to the sensor as standard) or increasing the sampling frequency (number of pixels per unit area) i.e. more megapixels with the same lens.

  • Compression artefacts, when an image is stored as a JPEG. Can be reduced by selecting the highest quality setting for JPEGs or shooting raw.

  • Hot pixels, stuck pixels, dark pixels. Sensor elements that always give either zero or the maximum possible response.


The term "colour noise" describes how the noise manifests itself - it's not a source of noise like the above. Colour noise refers to random variations in the colour of pixels, not just in their brightness. Colour noise is easy to remove since the eye is less sensitive to spatial variations in colour, the loss of detail due to noise reduction is less noticeable.

Again "high frequency noise" refers to another characteristic, the spatial frequency, or how close together the peaks of the noise are.

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3  
Nice answer... it's worth noting that these noise sources (except JPEG artifacts) are present in the raw data, which is one color (red, green, or blue) per pixel. So the exact appearance of the noise (as "color noise," etc.) depends also on the details of the demosaicing algorithm that combines data from multiple pixels to form a color image. –  coneslayer May 10 '11 at 16:51
    
Wow, I thought I understood noise before, but that was quite far over my head. –  dpollitt May 10 '11 at 17:54
    
@coneslayer-- actually, unless you have a Foveon sensor, the bayer pattern will limit the measured information from the chip to one color per pixel. Subsequent interpolation of that grid to produce a three-color image will exacerbate any noise that's produced, but is in and of itself not a source of noise. It does mean that, practically speaking, the noise isn't limited to a single pixel, so removing it can be painful (from a theoretical perspective). –  mmr May 10 '11 at 18:15
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curse the foveon! Without that sensor I wouldn't have to say [most] every time I talk about bayer sensors or demosaicing! It is still possible to have noise sources which show up as monochrome or colour that are not the result of demosaicing, e.g. noise in the near visible infra-red spectrum is more likely to excite pixels behind red filters. –  Matt Grum May 10 '11 at 18:22
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@Nir Yeah its part of shot niose. Raising the ISO doesn't create noise as many people think (except a tiny amount due to the amplification process) it merely reveals noise as it amplifies the noise as well as the signal. When you increase the ISO you tend to use a shorter exposure or smaller aperture, so you get more shot noise. If you drop the ISO whilst keeping the shutter and aperture constant you get more noise not less! –  Matt Grum May 11 '11 at 15:40

You only need to know two kinds of noise which come with all cameras, visible at higher ISOs.

  • Chroma Noise - colored blotched type of noise.
  • Luminance Noise - grain, film like noise.

To minimize such noise, try not to shoot at higher ISO's which start to deteriorate image quality (different for each camera). Using fast lenses such as f2.8 or F1.4 could help you in low light situations not to push ISO too high to keep the shutter speed high enough. If the scene is way too dark, the only way to minimize noise is to use a flash.

Lastly, there is software such as Noise Ninja or Dnoise which can help get rid of some noise for you without sacrificing IQ.

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Just frankly not true - esp the "visible at higher ISOs" part and the whole D3s comment. –  rfusca May 10 '11 at 20:07
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I'm not going to upvote this atm., but I find the (over)simplified statement about the two kinds of noise not too bad. If the stuff about the Nikon is removed, I think this could be a helpful if oversimplified answer :-) –  Martin May 11 '11 at 7:09

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